I agree with Big Jim...I would definitely recommend adding to the schools you currently have before thinking of adding any other species of fish. IME, zebra danios are more comfortable with company, however I've *never* seen mine swim together as a group. They spend their day chasing each other in all levels of the tank. That being said, I'd still recommend adding at least two. The Black skirts on the other hand often school beautifully together and keeping them in large groups cuts down on aggression and fin nipping. I'd highly recommend adding at least five more of those. I have a group of 7 in my 37 gal
and they are very peaceful and happy. Your corys will also be happy with more tankmates. I'd consider adding 1-2 more of those as well.